As is well known, in a terminating set of a telephone communication channel connecting a two-wire circuit connected to a subscriber telephone set, constituting a two-way transmission channel for that subscriber, and a four-wire circuit, constituting for the subscriber separate transmit and receive channels, there is a stray transmission path from the receive to the transmit channel. Thus part of the signal on the receive channel is retransmitted to the other, distant end subscriber communicating with the first-mentioned, near end subscriber. If the transmission of this part of the signal, known as the echo signal, the stray path being known in the echo path, constitutes a problem, which is the case with very long links over which the distant end subscriber perceives his own words as an echo, and on links in networks using concentrator type systems to reduce traffic throughout, in which circumstances the efficiency of such systems is considerably degraded, the four-wire circuit is fitted with a half-echo suppressor which blocks transmission of an echo signal on the transmit channel in the absence of speech signals from the near end subscriber but permits transmission of a speech signal from the near end subscriber whether or not an echo signal is superimposed on such signals. The four-wire circuit naturally incorporates another half-echo suppressor for the echo signals generated in the terminating set providing the connection between the four-wire circuit and the two-wire circuit connected to the telephone set of the distant end subscriber.
One way of detecting whether a signal present on the transmit channel consists only of the echo signal corresponding to a signal present on the receive channel, or if it also includes speech signals from the near end subscriber, is to compare the level of the signal on the transmit channel with a detection threshold which is a function of the signal level on the receive channel, and to evaluate the attenuation of the echo signal on the transmit channel relative to the signal on the receive channel, which is the attenuation of the echo path between the output (or possibly input) of the half-echo suppressor on the receive channel and the input of the half-echo suppressor on the transmit channel. The real value of this attenuation depends in particular on the specifications of the two-wire circuit connected to the terminating set. This attenuation may vary to a significant extent from one communication channel to another in the same network. Measuring this attenuation in a half-echo suppressor in operation provides for adapting its detection threshold to allow for such variations, so as to obtain sure and fast detection of the presence of a near end subscriber signal superimposed on the echo signal.
To carry out this measurement on the basis of the signals at both ends of the echo path in question, the propagation time on the echo path must be allowed for, as the signals are random in nature. This propagation time or echo delay is a function of the distance between the terminating set and the half-echo suppressor and, in some circumstances, on the specifications of other equipments such as filters on the transmission path. The echo delay varies from one communication channel to another for the same half-echo suppressor and may have a value of up to a few dozen milliseconds. A further factor must be taken into account when making such measurements, in that the near end subscriber speech signal may at certain times (when both subscribers are speaking) be superimposed on an echo signal on the transmit channel.
To overcome this problem, it has already been proposed to detect the maximum absolute values of the signals at both ends of the echo path, in time intervals of the same duration equal to the maximum echo delay, the maximum value detected for each of these signals in each time interval being stored in memory through the next time interval, the system determining at the end of each time interval the ratio of the larger of the two maximum values detected in the interval in question and the preceding interval for the signal at the output end of the echo path and the maximum value detected in the preceding interval for the signal at the input end of that path. The value of this ratio, previously peak limited to values exceeding 1 to minimise error at times when both subscribers are speaking and the distant end subscriber is not speaking, is the estimated value of the complement of the echo path attenuation.
This method generally gives a value less than the real value of the echo path attenuation, even when only the distant end subscriber is speaking.
Preferred embodiments of the present invention enable this attenuation to be measured more accurately without the use of significantly more complex circuitry.